The case study presented in this paper describes the thinking-in-action of a first grade teacher who, within the natural setting of her classroom, was constructing a new way of teaching language arts by changing to a whole language approach. In contrast to traditional implementation models, this study highlights the constitutive nature of innovation and accounts for the teacher's beliefs and values, her established version of practice, and her interpretation of the innovation. Data gathering and analysis occurred simultaneously and were based on observations of classroom activity, planning sessions, interviews, journal notes, and audio and video tapes of lessons. The bulk of the document deals with the composition of a new way of teaching described as "worldbuilding" that included: a new world of interaction between teacher and student with innovation constituted in the interaction; the initiation of "small conversations" composed of four worldbuilding patterns (participating as a learner/reader, naming actions, assuming possibilities, and changing boundaries); a view of the world that draws children in through patterned thought and action; and the ability of the teacher to interpret whole language through the lenses of her beliefs, values, and goals. A diagram of changing practice as a worldbuilding process is provided. (Contains approximately 75 references.) (LL)